Elastic cord



Jan. 25, 1955 .1. E. LILIENFELD ELASTIC CORD INVEN TOR. um 71/: 15 L IL //vF4 0 A770/PN'EY United States Patent ELASTIC CORD Julius Et Lilieuteld, St. Thomas, V. I.

Application March 2, 1953, Serial No. 339,525

1 Claim. (Cl. 87-8) The invention relates to an elastic cord unit or support member of the nature of the conventional shockcord, e. g., as used for parachute and other shrouds, resilient upholstery, reinforcing windings, etc. The conventional shockcord embodies a core formed by a bundle of parallel, substantially elastic filaments, such as rubber filaments, integrated with a sheath or textile casing such as of cotton braiding which tightly contains said core of filaments and is of limited deformability but insignificant elasticity. Both the elasticity and the extensibility of each constituent elastic filament is substantially higher than that of the integrated cord unit.

The core of such conventional shockcord has a functional elastic characteristic analogous to an exponential function, inasmuch as the increment of strain per unit increment of stress increases with the load. This would make the shockcord unsafe for many applications except for the presence of the enclosing deformable and substantially inelastic sheath which imposes a limit to the extension of the integrated cord unit and so provides for a functional behavior roughly analogous to a logarithmic function and thus for reduction of said strain increment to a lower order of magnitude before the member dangerously approaches its yield point.

An object of the present invention is to provide an elastic cord unit or support member without the provision of an envelope or sheath to limit its extension, and so to consolidate the functions of core and sheath in a unitary member that it shall possess a functional characteristic analogous to a linear function. If additional filaments are provided, extensibility is not materially decreased, the breaking point, however, is increased.

A further object of the invention is to provide sturdy elastic cords with rapid recovery properties and so designed as to support given loads with a specified elongation.

Another object of the invention is to provide an elastic cord of materially less cross-section and reduced volume and weight as well as lower cost as compared with conventional elastic cords designed for like loads.

Still another object of the invention is to construct an elastic cord of material chemically inert and capable of withstanding temperature variations over a large range, particularly of material usable at low temperatures and having, also, a smooth surface and not being readily injured upon kinking.

In carrying out the invention, the novel elastic cord is formed solely of a plurality of filaments less elastic than the cord unit into which they are integrated. To this end, said filaments are made of a suitable polymerized organic compound, of which the elasticity modulus is of a specifically circumscribed order of magnitude. In accordance with the invention, the filaments are made to control the functional behavior of the novel cord in the aforesaid manner by tight braiding along a path of a broadly helical character and, for optimum performance, of a pitch as high as feasible. Thereby, an integrated coreless cord is produced having an elastic characteristic analogous to a linear function; and, for superior elastic qualities, a plurality of such braided members may be formed or rebraided into a compound braid.

As a suitable material for the monofilaments, I have found oriented polyethylene most advantageous, although other suitable synthetic plastics such as some of the nylons, Orlons, Dacrons, etc., may serve, e. g., when a superior tensile strength is the dominant requirement. In any event, the material should possess adequate tensile "ice 2 strength, only insignificant set, rapid elastic recovery, good elastic shear resistance to torsion, and low mutual friction warranted by substantially smooth contacting surfaces. Also, the material should not easily be injured by kinking of the same.

The nature of the invention, however, will best be understood when described in connection with the ac/ companying drawings, in which:

Fig. l is an elevational view of the novel cord unit with portions broken away to disclose the component filaments of which it is constructed, the filaments being shown loosely braided for clearness.

Fig. 2 is an elevational view of a finished, length of the novel elastic cord unit, on an: enlarged. scale; and. Fig. 3 is a transverse section through the cord, taken. on the line 33, Fig. 2 of the drawings.

Fig. 4 is an elevational view illustrating a modification in the cord unit construction.

Referring to the drawings, more particularly Figs. 1-3 thereof, a plurality of monofilaments 10 are formed into a braid 11 in the conventional manner, care being exercised, however, that the braiding be, in accordance with the invention, of the highest feasible pitch and that it be effected as tightly as possbile. While but three filaments have been illustrated as the sole components of the completed cord unit which is shown more clearly in Figs. 2 and 3 of the drawings, it is to be understood that any larger number of filaments may be thus braided together.

The elastic nature of the material of which the monofilaments are composed is of prime importance, especially if the condition of relatively small cross-section is to be complied with, inasmuch as the modulus of elasticity thereof should be within the interval between orders of magnitude of 10 and 10 The apparent reason for this limitation is that the use of material having a higher modulus would result in a too high resistance to torsional shear so that the assembled cord unit would not have sufiicient elasticity; whereas, with material of lower modulus, as in the case of highly resilient rubber, the desired effect would not be-produced as it readily yields to applied pressure. The exact theory for the behavior of the novel cord unit does not, however, appear to be available in the art.

The filament material should have, also, insignificant set, rapid elastic recovery and good elastic shear resistance. The bare filaments utilized, moreover, are of smooth surfaces of low mutual friction and not easily injured by kinking.

I have found the polymerized organic compound known commercially as oriented polyethylene to be particularly suitable as material for the monofilaments. For example, a shockcord constructed of three of such filaments each having a diameter of 0.036" is safely usable to provide for an elongation of approximately 25% of its original length, whereas the elongation of an individual filament thereof may not exceed 10% also, a load of at least 15 lbs. may be sustained by such cord unit.

In the case of the braiding into an integrated unit of a greater number of filaments, for example seven monofilaments, say of 0.021 diameter each, I have found that the elasticity of such cord unit is approximately 25% less than that in the case of the unit of the former example, although in that instance the sum total of the cross-sectional areas of the component filaments is greater in the ratio of approximately 3 to 1. This result, when considered from the angle of the knowledge available in the art, was unpredictable.

While I have found the polyethylene to be very satisfactory for the purpose, other polymerized compounds such, for example, as some of the nylons, Orlons and Dacrons are suitable, as where superior tensile strength, or high melting point, etc., is desired; and I therefore do not wish to be confined to the use of polyethylene.

After braiding of the filaments as aforesaid has been completed, the free ends of the filaments at each end of a length of braiding 11 are united to prevent unravelling, as by welding or fusing the same together, as is indicated at 12, or by providing a knot 13 thereof, or otherwise eifecting a substantial junction, as is well understood. These ends may receive suitable terminal connections (not shown) whereby the cord may be connected between, for example, the canopy of a parachute and the load-supporting harness thereof (not shown), and as is well understood. In order to set the braided cord components at the desired pitch, I have found it desirable, also, to anneal the [braided cord, as by heat treating the same at approximately 220 F.

Other uses of the cord are known; for example, it may be utilized in resilient upholstery, and as a reinforcing winding, etc.

In instances where the shockcord is to be subjected to particularly great stresses and maximal elongation desired, a pluralitybut not less than threeof the braided units may be braided together similarly to the braiding of individual filaments to provide a compound unit thereof. By this expedient, a combination of extremely high elasticity and flexibility, as Well as maximal elongation, may be obtained so that substantial stresses may be accommodated. Reference being had to Fig. 4, three braided filaments 15, 16 and 17 are indicated and these in turn are braided together in the manner aforesaid to provide the novel shockcord 18.

I claim:

A flat-braided coreless shockcord consisting of at least three bare filaments tightly braided of a material of insignificant set having a modulus of elasticity within the interval between orders of magnitude of 10 and 10 the braiding being of maximal feasible pitch and the filaments being in intimate contact with one another whereby elongation of the unit under load is linear within the range of linearity of the constituent filaments.

References Cited in the file of this patent UNITED STATES PATENTS 315,438 Nicholson Apr. 7, 1885 2,001,676 Gastrich May 14, 1935 2,407,634 Du Pont Sept. 17, 1946 2,418,702 Du Pont Apr. 8, 1947 2,505,707 Du Pont Apr. 25, 1950 2,560,563 Fink et al July 17, 1951 

